A unique instrument for the biggest telescope

Resolved stellar populations enable us to explore the star-formation and chemical-enrichment histories of galaxies, providing direct constraints on galaxy formation and evolution models. Current facilities limit precise chemical abundances and stellar kinematics measurements to the Local Group. However, to fully assess the diversity of galaxy populations we need to move to a broader range of galaxies in the Local Volume, from the edge of the Local Group out to Mpc distances.

Metallicities studies of large number of stars in the outer halo of external galaxies (e.g. the Sculptor Group) would provide an extraordinary tool to characterize their past star formation and evolution history. The CaT will likely be the most used metallicity diagnostic similarly to what done with Gaia, but also the Mg I b triplet or the G-band could be valid alternatives, but it would require to go down at least to 0.41 mm. For basic metallicities measurements a R³5000 should be sufficient, given an adequate S/N (³20).

A powerful tool to reconstruct the formation of a given stellar system consist in determining the evolution of the chemical enrichment of that system. In order to obtain it once needs to measure chemical abundance of many elements (alpha-elements, s- and r- process, etc…). The goal is to perform a detailed chemical tagging (i.e., identifying peaks in the abundances space), which would require accessing to as many elements as possible, which translates into a wide spectral coverage from the optical to the NIR (Bland-Hawthorn et al. 2010, ApJ, 721, 582, Ting et al. 2012, MNRAS, 421, 1231, Hogg et al 2016, ApJ, 833, 262). For example,  the 0.8-1.8 μm range can provide direct abundance estimates of elements such as iron-peak (Fe, Mn, Cr, Co, Ni), α-group (Mg, Si, Ti, Ca), other light elements (atomic C, S, Na, Al, K) and Sr (an s-process element). These are critical diagnostics to reconstruct the chemical-enrichment history of stellar systems, in both dwarf and red giant/supergiant stars (e.g. Allende-Prieto et al. 2008). In particular, the simultaneous observation of CO, CN and OH lines in the H-band can provide robust abundance estimates of C, N and O in cool stars.

In addition, a high spectral resolution will be required to enable abundaces measurement using lines that at lower resolution would be blended. High-resolution spectroscopic surveys as APOGEE (IR, R~20,000) and GALAH (optical, R=28,000) have enabled the analysis of over 15 elements. Recent simulations for MOONS of quantitative analysis in the H-band, using the latest model-atmosphere-fitting techniques, have concluded that R~18,000 is sufficient for the global abundance precision required for each species (with R ≥ 20,000 still preferable/desirable).

With current and upcoming instrumentation at 8-10m telescopes, precise chemical tagging is limited to stars on the upper red-giant branch in the Magellanic Clouds, and to even more luminous red super-giants or stars on the asymptotic giant branch in Local Group galaxies. MOSAIC at the ELT would significantly extend these studies to larger distances and less luminous stars.